Block copolymers comprise sequences (“blocks”) of monomer units, covalently bound to sequences of unlike type. The blocks can be connected in a variety of ways, such as A-B in diblock and A-B-A triblock structures, where A represents one block and B represents a different block. In a multi-block copolymer, A and B can be connected in a number of different ways and be repeated multiply. It may further comprise additional blocks of different type. Multi-block copolymers can be either linear multi-block or multi-block star polymers (in which all blocks bond to the same atom or chemical moiety).
A linear block copolymer is created when two or more polymer molecules of different chemical composition are covalently bonded in an end-to-end fashion. While a wide variety of block copolymer architectures are possible, most block copolymers involve the covalent bonding of hard plastic blocks, which are substantially crystalline or glassy, to elastomeric blocks forming thermoplastic elastomers. Other block copolymers, such as rubber-rubber (elastomer-elastomer), glass-glass, and glass-crystalline block copolymers, are also possible and may have commercial importance.
One method to make block copolymers is to produce a “living polymer” Unlike typical Ziegler-Natta polymerization processes, living polymerization processes involve only initiation and propagation steps and essentially lack chain terminating side reactions. This permits the synthesis of predetermined and well-controlled structures desired in a block copolymer. A polymer created in a “living” system can have a narrow or extremely narrow distribution of molecular weight and be essentially monodisperse (i.e., the molecular weight distribution is essentially one). Living catalyst systems are characterized by an initiation rate which is on the order of or exceeds the propagation rate, and the absence of termination or transfer reactions. In addition, these catalyst systems are characterized by the presence of a single type of active site. To produce a high yield of block copolymer in a polymerization process, the catalyst must exhibit living characteristics to a substantial extent.
Butadiene-isoprene block copolymers have been synthesized via anionic polymerization using the sequential monomer addition technique. In sequential addition, a certain amount of one of the monomers is contacted with the catalyst. Once a first such monomer has reacted to substantial extinction forming the first block, a certain amount of the second monomer or monomer species is introduced and allowed to react to form the second block. The process may be repeated using the same or other anionically polymerizable monomers. However, ethylene and other α-olefins, such as propylene, butene, 1-octene, etc., are not directly block polymerizable by anionic techniques.
Recently, a method has been described for a process of catalytically making block copolymers with a controlled sequence distribution in publication WO2007/035485. As discussed in this publication, it has long been known that polymers containing a block-type structure often have superior properties compared to random copolymers and blends. For example, triblock copolymers of styrene and butadiene (SBS) and hydrogenated versions of the same (SEBS) have an excellent combination of heat resistance and elasticity. Other block copolymers are also known in the art. Generally, block copolymers known as thermoplastic elastomers (TPE) have desirable properties due to the presence of “soft” or elastomeric block segments connecting “hard” either crystallizable or glassy blocks in the same polymer. At temperatures up to the melt temperature or glass transition temperature of the hard segments, the polymers demonstrate elastomeric character. At higher temperatures, the polymers become flowable, exhibiting thermoplastic behavior. Known methods of preparing block copolymers include anionic polymerization and controlled free radical polymerization. Unfortunately, these methods of preparing block copolymers require sequential monomer addition with polymerization to relative completeness and the types of monomers that can be usefully employed in such methods are limited. For example, in the anionic polymerization of styrene and butadiene to form a SBS type block copolymer, each polymer chain requires a stoichiometric amount of initiator and the resulting polymers have extremely narrow molecular weight distribution, Mw/Mn, preferably from 1.0 to 1.3. That is, the polymer block lengths are substantially identical. Additionally, anionic and free-radical processes are relatively slow, resulting in poor process economics, and not readily adapted to polymerization of α-olefins.
It would be desirable to produce block copolymers catalytically, that is, in a process wherein more than one polymer molecule is produced for each catalyst or initiator molecule. In addition, it would be highly desirable to produce copolymers having properties resembling block copolymers from olefin monomers such as ethylene, propylene, and higher alpha-olefins that are generally unsuited for use in anionic or free-radical polymerizations. In certain of these polymers, it is highly desirable that some or all of the polymer blocks comprise amorphous polymers such as a copolymer of ethylene and a comonomer, especially amorphous random copolymers comprising ethylene and an α-olefin having 3 or more carbon atoms. Finally, it would be desirable to prepare pseudo-block or block copolymers wherein a substantial fraction of the polymer molecules are of a controlled block number, especially diblocks or triblocks, but wherein the block lengths are a most probable distribution, rather than identical or nearly identical block lengths.
It would be useful to produce additional such block copolymers which are based on ethylene and α-olefins and have at least one low crystallinity hard block.